Planar array antennas when imposed to cover multiple directions, suffer from scan loss. Since the projected aperture decreases as the beam is steered away from the broadside position which is normal to the ground surface and centered to the surface itself, it follows then that broadside excitation of a planar array yields maximum aperture projection. Accordingly, when such an antenna is made to come off the normal axis, the projected aperture area decreases causing a scan loss which is a function of cosine having a value of 1 with the argument of zero radians (normal) and having a value of 0 when the argument is ##EQU1## ##EQU2##
The multiple antenna beams of a communication system may be generated through use of a planar or cylindrical array of antenna elements, by providing signals to the individual antenna elements with a predetermined phase relationship (i.e., a phased array). This phase relationship causes the signal simulcast from the various antenna elements of the array to destructively and beneficially combine to form the desired radiation pattern. There are a number of methods of beam forming using matrix type beam forming networks, such as Butler matrixes commonly used in prior art systems. Likewise, there are a number of methods of beam steering using matrix type beam forming networks that can be made to adjust parameters as directed from a computer algorithm. This is the basis for adaptive arrays.
When a linear planar array is excited uniformly to produce a broadsided beam projection, the composite aperture distribution resembles a rectangular shape. When this shape is Fourier transformed in space, the resultant pattern is laden with high level side lobes relative to the main lobe. The ##EQU3##
function is thus produced in the far-field pattern. In most practical applications these high level side lobes are an undesirable side effect.
Cylindrical arrays may be preferable to a planar array due to the symmetry of a cylindrical antenna array providing improved side lobe level control as each antenna beam may be substantially orthogonal to a portion of the broadside of a cylindrical antenna system. Accordingly, if adapted properly, such a system may be utilized to provide superior antenna beam forming, i.e., substantially reduced scan loss and side lobes, for example, over that provided by a planar array.
Interference experienced in wireless communication, such as may be caused by multiple users of a particular service and/or various radiating structures of a service or different services providing communication coverage within the same or different geographical areas, may be controlled, at least to a limited extent, through antenna beam side lobe control. Through side lobe control, substantially only desired areas may be included in the antenna beam, thus avoiding energy radiated from undesired directions in the receive link and radiating energy in undesired directions in the transmit link. However, often in the past antenna beam side lobe control has been accomplished through the removal of antenna elements in outer columns of the phased array. However, this solution is generally not possible in a cylindrical array as the outer columns of one beam are the inner columns of another beam and, thus, removal of these elements would adversely affect beam formation.
As the use of wireless communications increases, such as through the deployment of new services and/or the increased utilization of existing services, the need for interference reduction schemes, such as techniques for reducing the aforementioned side lobes, becomes more pronounced. Further control of interference and improvement in communications may be had through steering antenna beams not only in the azimuth, but also in the elevation, to direct an antenna beams to a user and/or to isolate other user's signals.
For example, in code division multiple access (CDMA) networks a number of communication signals, each associated with a different user or communication unit, operate over the same frequency band simultaneously. Each communication unit is assigned a distinct, pseudo-random, chip code which identifies signals associated with the communication unit. The communication units use this chip code to pseudo-randomly spread their transmitted signal over the allotted frequency band. Accordingly, signals may be communicated from each such unit over the same frequency band and a receiver may despread a desired signal associated with a particular communication unit.
However, despreading of the desired communication unit's signal results in the receiver not only receiving the energy of this desired signal, but also a portion of the energies of other communication units operating over the same frequency band. Accordingly, CDMA networks are interference limited, i.e., the number of communication units using the same frequency band, while maintaining an acceptable signal quality, is determined by the total energy level within the frequency band at the receiver. Therefore, it is desirable to limit reception of unnecessary energy at any of the network's communication devices.
In the past, interference reduction in some wireless communication systems, such as the aforementioned CDMA cellular systems, has been accomplished to an extent through physically adjusting the antenna array to limit radiation of signals to within a predefined area. Accordingly, areas of influence of neighboring communication arrays may be defined which are appreciably smaller than the array is capable of communicating in. As such, radiation and reception of signals is restricted to substantially only the area of a predefined, substantially non-overlapping, cell.
Changes in the environment surrounding a communication array or changes at a neighboring communication array may require adjustment of the radiation pattern of a particular communication array. Specifically, seasonal changes around a base transceiver station (BTS) site can cause changes in propagation losses of the signal radiated from a BTS. For example, during fall and winter deciduous foliage loss can cause a decrease in signal path loss. This can result in unintentional interference into neighboring BTS operating areas or cells as the radiation pattern of the affected BTS will effectively enlarge due to the reduced propagation losses.
Likewise, an anomaly affecting a neighboring BTS may cause an increase in signal path loss, or complete interruption in the signal, therefore necessitating the expansion of the radiation patterns associated with various neighboring BTSes in order to provide coverage in the affected areas.
Previously, crews have had to be dispatched to purposely tilt BTS antennas up or down to minimize interference or provide coverage in neighboring areas. Likewise, crews have again had to be dispatched when the anomaly affecting the signal has dissipated or been resolved. Such adjustment is typically accomplished in concert with observation of field measurement, such as may be available from drive testing or by the results of operation statistical records. It becomes readily apparent that compensation for such anomalies, even occurring only seasonally, can be quite expensive. Furthermore, as the communication system grows in complexity, more such adjustments have to be made to bring the system back up to full operating capacity.
Additionally, it may be desirable to independently adjust the beams. For example, the aforementioned anomaly affecting radiation of signals may affect only certain antenna beams of an array and, therefore, only a subset of the antenna beams require adjustment. Likewise, adjustment of only a selected antenna beam in order to provide communication to a particular mobile communication unit may be desirable. However, it is not common for current systems to provide for the adjustment of individual antenna beams of an antenna array due to the complexity of adaptation of prior art feed networks in order to provide such per antenna beam steering. For example, adapting the aforementioned Butler matrixes of the prior art, which provide the phase progressions of multiple ones of the antenna beams formed by such a phased array system, to allow individual antenna beam elevation steering is very difficult, if not impossible.
Accordingly, a need exists in the art for a system and method providing improved antenna beam forming to direct communication signals to/from subscriber units substantially to the isolation of other signals.
A further need exists in the art for a system and method providing antenna beam forming utilizing efficient circuitry to enable establishing a phased array signal without unnecessary signal power losses such as associated with non-coherent combining in an antenna feed network.
A still further need exists in the art for a system and method providing individual elevation "down-tilt" of antenna beams providing illumination of a desired area in order to reduce interference and allow frequency reuse by additional such antenna systems.
A yet further need exists in the art for a system providing antenna beam side lobe control without unnecessarily compromising the ability to form adjacent antenna beams.